Regulation of intracellular pH (pHi) in response to various stimuli is a critical one among a number of cellular functions. A family of bicarbonate transporters is a major pHi regulator under physiological conditions in animal cells. Bicarbonate (HCO3−) transporters are divided into four groups according to their functions [Boron, W. F. et al., J. Exp. Biol., 200:263–268(1997)]: Na+-independent Cl−/HCO3− exchanger (alternatively called an anion exchanger, AE), Na+-HCO3− cotransporter (NBC), K+-HCO3− cotransporter, and Na+-driven Cl−/HCO3− exchanger. Three AEs and three NBCs have been cloned and functionally characterized, but the molecular structure of the K+-HCO3− cotransporter and the Na+-driven Cl−/HCO3− exchanger have remained unknown.
A Na+-driven Cl−/HCO3− exchanger was first discovered in invertebrate neurons and was later found in vertebrate neurons as well as non-neuronal cells, including brain, vascular endothelial cells, sperm, kidney and pancreatic β-cells. Na+-driven Cl−/HCO3− exchanger is an intracellular pH regulator that transports extracellular Na+ and HCO3− into the cells in exchange for intracellular Cl−, thereby playing an important role in cellular alkalinization.
In pancreatic, β-cells, glucose is the most important physiological regulator of insulin secretion. Glucose metabolism induces an increase in intracellular pH in the pancreatic cells. It has been shown that this glucose-induced pHi rise is evoked primarily by the action of Na+-driven Cl−/HCO3− exchanger [Pace, C. S. et al., J. Membrane Biol., 73:39–43(1983)].
Na+-driven Cl−/HCO3-exchanger is thus an important intracellular pH regulator in various cells, but its molecular basis is not known. Analysis of the molecular structure and function of Na+-driven Cl−/HCO3− exchanger should be valuable not only for functional analysis of insulin secretion by pancreatic β-cells but also for screening as well as for drug designing based on its molecular structure aimed at the development of therapeutics of diabetes mellitus.
On the above background, the present invention has as its objective to clone Na+-driven Cl−/HCO3− exchangers, thereby obtaining their DNA for sequencing, providing cells of a different species expressing the DNAs, and determining the structure and function of the Na+-driven Cl−/HCO3− exchangers.